In the crushing of hard material, e.g., stone blocks or ore blocks, materials are frequently crushed that have an initial size of, e.g., 300 mm or less to a size of, e.g., approx. 0-25 mm by means of a gyratory crusher. An example of a gyratory crusher is disclosed in U.S. Pat. No. 4,566,638. Said crusher has an outer shell that is mounted in a frame. An inner shell is fastened to a crushing head. The crushing head is fastened to a shaft, which at the lower end thereof is eccentrically mounted and which is driven by a motor. Between the outer and the inner shell, a crushing gap is formed into which material can be supplied. Upon crushing, the motor will get the shaft, and thereby the crushing head, to execute a gyratory pendulum motion, i.e., a motion during which the inner and the outer shell approach each other along a rotary generatrix and retreat from each other along another diametrically opposite generatrix.
It is a common problem upon crushing of hard materials by means of a gyratory crusher that a number of material pieces have a substantially larger size than what the desired crushing gap can accept. As a consequence, these pieces are not crushed but remain above the crushing gap and block materials having smaller grain size from coming down into the crushing gap and be crushed. As a result, blockages may arise, which entail a capacity reduction and that a manual cleaning has to be carried out. In practice, the consequence will frequently be that an unnecessary wide crushing gap has to be chosen so that even the large material pieces can come down into the crushing gap. However, this leads to a deteriorated size reduction of the supplied material and an unfavourable wear pattern of the shells.